PostgreSQL is a very powerful open source database management system. It's licensing is clear in that you can use if for whatever you like. That includes using it in a commercial product.

What we like about PostgreSQL:

• No licensing issues.
• Standards  based.
• Highly scalable.
• Fast and powerful. Can handle billions of records.
• Publicly available support is good.

What we don't like aboiut PostgreSQL:

• Is challenging to install, update, and maintain.

For more information on PostgreSQL visit the webpage here: http://www.postgresql.org/

For a comparison with MySQL go here: http://www.wikivs.com/wiki/MySQL_vs_PostgreSQL